1. Field of the Invention
The present invention relates to an inductor element for noise suppression that is used as an electronic circuit component and, in particular, it relates to an inductor element that imparts a noise suppressing effect in the GHz band.
2. Discussion of Background
Of the noise that enters an electronic circuit or is generated in an electronic circuit, in many cases a problem is presented by the component that is of a higher frequency than the signal frequency, and the normal countermeasures taken against such noise are intended to remove this component. A low pass filter or an electronic circuit with a similar effect is widely used for that purpose. These take advantage of the frequency dependency of impedance matching or mis-matching and on the high frequency side, filter characteristics are achieved by reflecting the signal. However, in such a case, the unnecessary high frequency component is returned to a preceding stage and this may result in, for instance, unexpected oscillation or the like in the circuit. In principle, therefore, it is desirable to remove such unnecessary frequency components through absorption.
Low pass type elements in the prior art that take advantage of absorption include ferrite bead elements. A ferrite bead element is an inductor element that uses ferrite for its core. As with normal inductor elements, the impedance increases as the frequency becomes higher and, at a specific frequency, the loss imparted by the ferrite material used for the core becomes pronounced. By matching the frequency of the noise to be removed with the frequency of the loss in the core, noise suppression through absorption is achieved.
However, the loss of ferrite occurs in the MHz band or, at the highest, at a few GHz, although this varies depending upon the composition of the ferrite, and if an inductor element is constituted with ferrite, effective noise suppression cannot be achieved in the GHz band.
U.S. Pat. No. 4,297,661 discloses a high pass filter that is constituted by employing a microstrip structure with ferrite. This high pass filter takes advantage of a phenomenon in which the absorption that occurs on the low range side disappears on the high range side.
Schiffres (IEEE Trans. Electron. Magn. Compot. EMC-6 1964, pages 55 to 61) sets out an example of an element using ferrite in the form of a coaxial transfer line, but this example aims at acquisition of characteristics mainly in the MHz band and does not disclose transmission characteristics in the high frequency range at or above the GHz band. It is assumed that similar transmission takes place in the GHz band.
In either of the prior art technologies described above, it is difficult to obtain a noise suppressing element capable of noise suppression in the GHz band by using ferrite only and combining ferrite with other materials has been suggested. As an example of such a combination, an attempt for noise suppression in the high frequency side through the combination of a non-magnetic material with absorption on the high range side with a ferrite has been reported.
This example was featured in the art by Schlicke (IEEE Spectrum 1967, pages 59 to 68) and the art disclosed by Bogar (Proc. of IEEE 67 1979, pages 159 to 163). In these technologies, a structure in which a ferrite and a dielectric body are provided coaxially at a portion of an insulator is employed. In addition, the art disclosed by Fiallo (IEEE Transactions of Microwave Theory and Techniques 1994, pages 1176 to 1184) reports on a microstrip structure in which a ferrite and a dielectric body are combined.
However, the elements disclosed in the prior art publications mentioned above, have complicated shapes, and they cannot be inserted in a circuit as easily as ferrite beads. In particular, while ferrite beads do not require grounding, the elements disclosed in the prior art publications require electrical grounding as well as signal lines.
The inventors of the present invention noted that a compound material that is achieved by combining ferromagnetic metal particles and resin imparts an electric wave absorbing effect in the GHz band. Examples of noise suppression that employ a magnetic metal particles-resin compound material are described below, although no disclosure of an inductor element for noise suppression in the GHz band, as in the present invention, is set forth in these examples.
For instance, in U.S. Pat. No. 4,146,854, an attenuating element is constituted with ferrite beads in combination with an electric wave absorbing body (a metal-resin compound material). In addition, in Japanese Unexamined Patent Publication (KOKAI) No. 127701/1992, an electric wave absorbing material is employed in a portion of a non-magnetic microstrip line. These two technologies feature an electric wave absorbing body used in a secondary capacity to suppress the excess high frequency component which could not otherwise be absorbed.
U.S. Pat. No. 4,301,428 discloses a technology for suppressing high frequency noise by using a metal-resin compound material with a suitable resistance value for a coaxial line and a signal line of a balanced line, and using a metal-resin compound material with an insulating property for a covering member. However, if a signal line is made to have an electrical resistance value, attenuation of the signal components will occur as well as suppression of the noise component and, therefore, this poses a problem when handling a weak signal. In addition, this example of prior art discloses a technology for electric cables and does not include instances in which the technology is employed in a circuit element.
At the same time, compound materials constituted of ferrite and resin are widely used as electric wave absorbing bodies. They are employed in these cases mainly for the purpose of absorbing electric waves radiated in the air and, therefore, the object is different from that of the present invention, which employs such a material for a circuit element.
In addition, compound materials constituted with iron particles and resin have been in use as a core material in a coil, i.e., the so-called dust core, for a long time. In this case, it is desirable to minimize the absorption loss since the material is used to constitute an inductor element in a circuit, and therefore, the attitude is just the contrary of that in the present invention, which actively takes advantage of the loss of the material.
Furthermore, in Japanese Patent Application No. 209586/1994 and Japanese Patent Application No. 9333/1995, and the publication in Microwaves & RF, February 1995, pages 69 to 72, the inventors of the present invention have disclosed a noise suppressing element for the GHz band employing a material similar to that in the present invention. What characterizes this element is that a grounding electrode is provided as well as a signal line to constitute a type of transmission path so that the characteristic impedance of the element can be matched with the characteristic impedance of the circuit from the passing band through the blocking range. It aims to minimize the reflection to absorb efficiently in the blocking range.
In this case, the full effect is realized when the impedance of the circuit to which the invention is applied is constant and there is a stable grounding pattern in the vicinity. However, if the characteristic impedance of the circuit is unstable due to a circuit-related reason or there is no grounding pattern nearby, it is difficult to take advantage of its noise suppression feature.
As has been explained, ferrite beads achieve simple and advantageous noise suppressing elements that do not require grounding, but they are not effective in the GHz band. In addition, while some elements that aim for noise suppression in the GHz band have been disclosed, they are not as simple or convenient as ferrite beads. Thus, realization of an inductor element with a structure similar to that of ferrite beads which provide a noise suppressing effect in the GHz band is eagerly awaited.
Furthermore, while methods in which ferrite is used in combination with a dielectric body and in which an electric wave absorbing body is employed secondarily have been disclosed, they pose problems such as requiring a grounding electrode and having complicated structures.